Declining Recruitment and Growth of Shovelnose Sturgeon in the Middle Mississippi River: Implications for Conservation

Abstract

To determine how populations of shovelnose sturgeon Scaphirhynchus platorynchus are changing and may respond to poor environmental conditions, current commercial harvest of black-egg (sexually mature) females, and incidental mortality of males, we require annual information about sex-specific age structure as it relates to the recruitment of new cohorts. We sampled shovelnose sturgeon by use of gill nets (5-cm bar mesh) monthly during 2002-2006 in the Middle Mississippi River between Cairo, Illinois, and St. Louis, Missouri. We compared patterns of size and age structure over time and projected age structure and population size into the future. Sex ratio in 2005-2006 was 1.14:1.00 (416 males and 363 females; P = 0.06), deviating from the 1:1 ratio that occurred in 2002-2003. Annual mortality increased from 37% in 2002-2003 to 44% by 2005-2006. Female shovelnose sturgeon were larger than males. Across years, the population shifted toward longer, older fish, and growth in length declined. Recruitment declined through time (29% per year). If these trends continue and if immigration from nonharvested populations is limited, population density may decline by an order of magnitude within one decade. Under current conditions, resilience to harvest and environmental perturbations is probably limited.